Electrical stimulation therapy may be used for various forms of treatment. For example, stimulation therapy may be provided to address neurological issues such as chronic pain, tremors, and the like. In such an example, an implantable stimulation device is typically located in one location of convenience and is connected to electrical leads that are routed to a stimulation site such as within the brain, within the spinal column, within the pelvic region, or elsewhere. The electrical leads include electrodes that interface with the tissue at the stimulation site to deliver the stimulation signals from the stimulation device.
It may be useful to also sense physiological signals nearby the stimulation site. Such signals may be useful to tailor the stimulation therapy to the particular condition being treated and/or to better understand the response of the tissue nearby the stimulation site to the stimulation signals. Sensing physiological signals may be especially useful where the stimulation therapy may be at least partially controlled based on the physiological signals.
In order to provide the stimulation signals and to sense the physiological signals, conduction paths are present within the leads, and in lead extensions when present, in order to carry the signals between the electrodes at the stimulation or sensing site and the implantable medical device that includes the stimulation and sensing circuitry. Each of these conduction paths is formed by one or more fine wires that are insulated relative to the wires of the other conduction paths, where the collection of the one or more fine wires of a given conduction path may be linear or may be coiled along the length of the lead and lead extension. Due to age and patient activity, the insulation separating the conduction paths may deteriorate or may otherwise by damaged. Ingress of bodily fluids may also occur over time. Events such as these may result in signal leakage between conduction paths. Such leakage can result in potentially faulty therapy, particularly due to adverse effects of the leakage when attempting to sense relatively small neurological signals.
To further exacerbate this issue, the impedance of the leakage may be within the range of impedances considered to be normal for the conduction paths. Thus, if an impedance test from one conduction path to another is performed, where one of those conduction paths leaks to the other, then the impedance test will not reveal the leakage. Therefore, therapy may continue to be provided as if no leakage is present, and any issues presented by the leakage will continue to potentially adversely affect the therapy.